Fix the source buffer used by CopyTex[Sub]Image.

Fix the source buffer used by CopyTex[Sub]Image.

We were passing the framebuffer to the texture's copying method, where
the first render target was used, instead of the read buffer.

Change-Id: I344b4082f675b7e5fc6a73af31a6764ab4d3b6a3
Reviewed-on: https://swiftshader-review.googlesource.com/17049
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>